The present invention relates to emulsion polymers, and more specifically, emulsion polymers stabilized by a protective colloid.
Emulsion polymers are typically stabilized by surfactants or by a combination of surfactants and protective colloids e.g., polyinylalcohol or hydroxyethylcellulose (HEC). The use of such colloids has traditionally been restricted to the polymerization of highly reactive, hydrophilic monomers such as vinylacetate, e.g., polyvinylacetate, vinylacetate-ethylene (VAE) copolymers and vinylacrylics. Examples of the use of colloids in the emulsion polymerization of hydrophobic monomers such as styrene, butadiene are very rare and this probably is due to the extreme difficulty of getting stable emulsions in these systems. Latices stabilized by polyvinylalcohol or HEC have a unique rheology and tack properties compared to surfactant stabilized systems that make it attractive in application areas such as coatings and adhesives. Hence the interest in stabilizing other polymers using polymeric colloids is extremely high.
While numerous patents exist illustrating the use of polyvinylalcohol or other colloids in vinylacetate based polymers, it is just recently that two patents have issued that outline ways to make a polyvinyl alcohol stabilized butadiene copolymer emulsion. The first method, described in Japanese Patent 06179705 to Kuraray involves a two-step process in which the first step is to make a polyvinylalcohol end functionalized with a thiol (--SH) group. This is then used to stabilize butadiene-styrene emulsion in a subsequent step. The thiol functionalized polyvinylalcohol is made by polymerizing vinylacetate in the presence of, for example thiolactic or thioacetic acid and then hydrolyzing the resulting polymer.
The second patent, EP 0516360 to Lord Corporation proposes a butadiene polymer latex prepared by emulsion polymerizing butadiene with various monomers in the presence of polyvinylalcohol and a cosolvent such as methanol to facilitate the stabilization process. The methanol is subsequently removed from the latex by a stripping process.
The first process has the disadvantage that it involves two steps and that a separate functionalization step is required to make the polyvinylalcohol suitable for stabilizing a butadiene latex. The second process does not work without the cosolvent and hence the potential problems of solvent handling, recovery and recylcing issues become a major cost and manufacturing concern.
The current invention avoids both of these problems and presents a simple one-step approach to preparing polyvinylalcohol stabilized butadiene copolymers using conventional emulsion polymerization equipment. The advantages of the present invention relate to the use of a functional silane that facilitates the grafting/adsorption of the polyvinylalcohol onto the butadiene emulsion copolymer. Yet another advantage of the present invention is that the polymer emulsions produced can be stabilized exclusively by fully hydrolyzed polyvinylalcohol. Commercially available medium molecular weight, fully hydrolyzed polyvinylalcohols (such as ELVANOL.RTM.) can be used without stability problems. This is difficult to achieve even with polyvinylacetate and vinylacetate-ethylene copolymer emulsions because of the generally accepted observation that fully hydrolyzed polyvinylalcohol does not adsorb favorably on vinylacetate copolymer emulsion particles and is certainly unheard of in butadiene emulsion copolymers.